Medical electrical lead having bending stiffnesses which increase in the distal direction

ABSTRACT

An elongated coronary vein lead having a variable stiffness lead body and most preferably adapted to be advanced into a selected coronary vein for delivering a pacing or defibrillation signal to a predetermined region of a patient&#39;s heart, such as the left ventricle is disclosed. A method of pacing and/or defibrillating a patient&#39;s heart using the lead is also described. The method of pacing or defibrillating the heart includes advancing the coronary vein lead through both the coronary sinus and into a selected coronary vein of a patient&#39;s heart, connecting the lead to an electrical pacing source and applying electrical stimulation to a particular chamber of the patient&#39;s heart via the implanted lead. The lead includes a variable stiffness lead body that enhances the ability of the lead to be retained in a coronary vein after the lead has been implanted therein.

[0001] This patent application hereby incorporates by reference herein,in its entirety, co-pending U.S. patent application Ser. No. ______,filed Nov. ______, 1999 for “Medical Electrical Lead Having VariableBending Stiffness” to Smits having Attorney Docket No. P-8975.

FIELD OF THE INVENTION

[0002] The present invention relates to pacing and defibrillationmedical electrical leads. The present invention also relates to medicalelectrical leads adapted and configured for implantation within thecoronary sinus and coronary veins.

BACKGROUND OF THE INVENTION

[0003] Transvenously inserted leads for implantable cardiac pacemakershave conventionally been positioned within the right atrium or rightventricle of the patient's heart for pacing or defibrillating the rightatrium and/or right ventricle, respectively. While it is relatively safeto insert a pacing or defibrillation lead and its associated electrodesinto the right atrium or right ventricle, there is a reluctance toinstall a similar lead in the left ventricle because of the possibilityof clot formation and resulting stroke.

[0004] When a lead is implanted within a patient's circulatory system,there is always the possibility of a thrombus being generated andreleased. If the lead is positioned in the right atrium or rightventricle, a generated thrombus tends to migrate through the pulmonaryartery and is filtered by the patient's lungs. A thrombus generated inthe left atrium or left ventricle, however, would pose a danger to thepatient due to the possibility of a resulting ischemic episode.

[0005] Thus, in those instances where left heart stimulation is desired,it has been a common practice to use an intercostal approach using amyocardial screw-in, positive-fixation lead. The screw-in lead may,however, be traumatic for the patient. There are additional instanceswhen left ventricular pacing is desired, such as during bi-ventricularpacing. In U.S. Pat. No. 4,928,688, Mower describes an arrangement forachieving bi-ventricular pacing in which electrical stimulating pulsesare applied via electrodes disposed on a single pacing lead to both theright and left ventricular chambers so as to obtain a coordinatedcontraction and pumping action of the heart. The '688 patent alsodiscloses a split pacing lead having first and second separateelectrodes, wherein the first electrode is preferably introduced throughthe superior vena cava for pacing the right ventricle and the secondelectrode is introduced through the coronary sinus for pacing the leftventricle. Other electrode leads which are inserted into the coronarysinus have been described. For example, in U.S. Pat. No. 5,014,696 toMehra and U.S. Pat. No. 4,932,407 to Williams endocardial defibrillationelectrode systems are disclosed.

[0006] Still other leads and catheters have been proposed, includingthose described in the patents listed in Table 1 below. TABLE 1 U.S.Pat. No. Title 5,951,597 Coronary sinus lead having expandable matrixanchor 5,935,160 Left ventricular access lead for heart failure pacing5,931,864 Coronary venous lead having fixation mechanism 5,931,819Guidewire with a variable stiffness distal portion 5,925,073 Intravenouscardiac lead with wave shaped fixation segment 5,897,584 Torque transferdevice for temporary transvenous endocardial lead 5,871,531 Medicalelectrical lead having tapered spiral fixation 5,855,560 Catheter tipassembly 5,833,604 Variable stiffness electrophysiology catheter5,810,867 Dilation catheter with varied stiffness 5,803,928 Side access“over the wire” pacing lead 5,755,766 Open-ended intravenous cardiaclead 5,755,765 Pacing lead having detachable positioning member5,749,849 Variable stiffness balloon catheter 5,733,496 Electron beamirradiation of catheters to enhance stiffness 5,639,276 Device for usein right ventricular placement and method for using same 5,628,778Single pass medical electrical lead 5,605,162 Method for using avariable stiffness guidewire 5,531,685 Steerable variable stiffnessdevice 5,499,973 Variable stiffness balloon dilatation catheters5,437,632 Variable stiffness balloon catheter 5,423,772 Coronary sinuscatheter 5,330,521 Low resistance implantable electrical leads 5,308,342Variable stiffness catheter 5,144,960 Transvenous defibrillator lead andmethod of use 5,111,811 Cardioversion and defibrillation lead systemwith electrode extension into the Coronary sinus and great vein4,930,521 Variable stiffness esophageal catheter 4,215,703 Variablestiffness guide wire 08/794,175 Single Pass Medical Electrical Lead08/794,402 Single Pass Medical Electrical Lead with Gap Electrodes

[0007] As those skilled in the art will appreciate after having reviewedthe specification and drawings hereof, at least some of the devices andmethods discussed in the patents of Table 1 may be modifiedadvantageously in accordance with the present invention. All patentslisted in Table 1 herein above are hereby incorporated by referenceherein, each in its respective entirety.

[0008] Prior art coronary vein leads for heart failure applications(i.e., pacing leads) or sudden death applications (i.e., defibrillationleads) generally must be wedged in a coronary vein to obtain a stablemechanical position and to prevent dislodgment. While such anarrangement is generally acceptable for defibrillation leads (whichusually must be implanted with the distal tip thereof located near theapex of the heart), such is not the case for heart failure or pacingleads, where more basal stimulation of the heart is generally desired.Basal stimulation of the heart via the coronary vein, however, presentscertain difficulties because vein diameters in the basal area of theheart are large and generally do not permit the distal end or tip of apacing lead to be sufficiently well wedged therein.

[0009] Medical electrical leads suitable for implantation within theright atrium and/or right the ventricle are known in the art. Leadshaving J-shapes imparted to the distal ends thereof are likewise knownin the art. Such leads having J-shaped distal ends typically exhibitsubstantial bending stiffness at the distal thereof, and are most oftenconfigured for placement in the right atrium. It is typical that duringimplantation of such a lead having a J-shaped section at the distal endthereof that, once the lead has been placed within the right atrium, thelead is retracted slightly to impart a positive tip force to the distalend of the lead. Relatively small displacements of the lead in such amanner can result in large variations in the force exerted by the tip ofthe lead upon the atrial wall. It is therefore not uncommon for theforce exerted by the tip to either be excessive or to even becomenegative, in which event the distal end of the lead is suspended fromits own tines or other distally disposed positive fixation device. This,in turn, leads to mechanical instability of the positioning of thedistal section of the lead within the right atrium or the rightventricle.

[0010] Thus, there exists a need to provide a pacing or defibrillationmedical electrical lead which exhibits better mechanical stabilityfollowing implantation.

SUMMARY OF THE INVENTION

[0011] The present invention has certain objects. That is, the presentinvention provides solutions to one or more problems existing in theprior art. For example, various embodiments of the present inventionhave one or more of the following objects: (a) providing a medicalelectrical lead suitable for implantation in the right atrium or rightventricle which is not mechanically unstable once implanted therein; (b)providing a medical electrical lead which exhibits enhanced removabilityfollowing implantation and fibrosis; (c) providing a medical electricallead suitable for implantation within the right atrium or rightventricle which requires less time and effort to implant; (d) providinga medical electrical lead which exhibits reduced overall stiffness atthe distal end thereof; (e) providing a medical electrical lead, theimplantation of which exhibits decreased dependency on the longitudinalposition of the lead body thereof in the veins leading to the rightatrium or right ventricle; (f) a medical electrical lead wherein smalldislodgments occurring near the entrance of the lead in the vein nearthe anchoring sleeve do not lead to electrode tip dislodgment; and (g) amedical electrical lead wherein the width of the J-shape impartedthereto resulting from implantation within the right atrium or rightventricle may vary according to the distance between the electrodeposition and the location of the superior vena cava.

[0012] Various embodiments of the present invention suitable forimplantation within the right atrium or right ventricle possess certainadvantages, including one or more of the following: (a) exhibitingmultiple lead mechanical stability points which exhibit less dependenceon positive fixation mechanisms for proper positioning relative to priorart leads; (b) providing a lead whose retention within the right atriumor right ventricle is less dependent upon the particular shape ordiameter of such heart chambers and venous anatomy than prior art leads;(c) providing a lead which permits improved pacing electrode positioningwithin the right atrium or right ventricle; (d) providing a lead whichpermits lower pacing thresholds and improved sensing of intra-cardiacsignals; (e) providing a lead which exhibits improved acute and chronicpacing thresholds and sensing characteristics; (f) providing a leadwhich has no or reduced positive fixation mechanisms attached thereto;(g) providing a lead which may be implanted with an introducer ofreduced size; (h) providing a lead which improves chronic leadremovability thereof; (i) providing a straight lead which is easier,more reproducible and less expensive to manufacture; (j) providing alead which exerts a positive electrode tip pressure or force upon theside wall of the right atrium or right ventricle; (k) providing a leadwherein the tip pressure exerted thereby is less dependent on thespecific location of the lead body with respect to the venous anatomyleading into the atrium; (l) providing a lead wherein the depth of theplacement of the lead tip into the right atrial appendix may beselectively varied; and (m) providing a medical electrical lead having astiffness which varies as a function of axial distance adapted forspecific placement and stability within veins other than the coronarysinus and great cardiac vein, wherein the lead exhibits appropriatedistal curvatures and bending stiffnesses required for implantationwithin the hepatic vein, spinal column, sub-cutaneously, or in otherlocations within the human body.

[0013] Various embodiments of the present invention exhibit one or moreof the following features: (a) a distal section of a pacing ordefibrillation lead having variable bending stiffness adapted andconfigured to create a forward driving force of the lead when thevariable bending stiffness portion of the distal end of the lead issubjected to a bending moment resulting in sufficient curvature; (b) apacing or defibrillation lead having in a distal portion thereof avariable bending stiffness section in which the bending stiffnessincreases with respect to axial distance; (c) a medical electrical leadwhich owing to variations in bending stiffness along its axial directionimparts a positive tip force or a forward driving force to the lead, andwhere bending of the lead may preferentially take place along differentpredetermined bending planes (e.g., three dimensional bending alongmultiple preferred orientations); (d) a pacing or defibrillation leadwherein variations in bending stiffness are rotationally symmetric; (e)a pacing or defibrillation lead wherein bending stiffness isrotationally asymmetric to permit orientation of one or more electrodes,fixation means, or other lead features relative to the bending plane ofa bent or curved section; (f) a pacing or defibrillation lead exhibitingvariable stiffness over at least distal portions thereof and which isfurther characterized in having active or passive fixation features, orno such features, being unipolar or multi-polar, being a pacing orsensing lead, being a defibrillation lead, or having a combination ofpacing/sensing and defibrillation capabilities; (g) providing a leadcapable of implantation within the right atrium or the right ventricle;(h) providing a lead which may be implanted within the right atrium,right ventricle, the coronary sinus, any of the various and/or one ormore of the coronary veins; (i) providing a medical electrical leadhaving enhanced positive tip pressure exerted thereby to promote thetransfer of drugs released from the distal tip or a distal portionthereof into the cardiac wall; (j) providing a medical electrical leadhaving a side arm extending therefrom in a single pass multi-chamberlead, wherein the side arm is employed to pace or defibrillate the rightatrium, and wherein the size of the heart within which the lead isimplanted assumes less importance is respect of prior art leads becausethe lead body may assume a greater range of positions within thesuperior vena cava; (k) in a single pass multi-chamber lead, a medicalelectrical lead having a side arm extending therefrom for implantationwithin the right atrium, which side arm may be more easily located alongdistal portions of the lead body to facilitate orientation and locationof the ventricular electrode; and (l) a medical electrical lead employedin conjunction with a pulled wire for imparting curvature to the distalportion thereof to facilitate handling and prevent the electrode frombecoming dislodged during the implantation procedure. Methods of making,using, and implanting a lead of the present invention are alsocontemplated in the present invention.

[0014] These and other objects, features and advantages of the presentinvention will be readily apparent to those skilled in the art from areview of the following detailed description of the preferred embodimentin conjunction with the accompanying drawings in which like numerals inthe several views refer to corresponding parts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIGS. 1A and 1B illustrate two different embodiments of thepresent invention implanted within a human heart;

[0016]FIG. 2A illustrates parameters employed in modeling one embodimentof the present invention;

[0017]FIG. 2B shows results obtained using the modeling assumptions ofFIG. 2A;

[0018]FIG. 3A illustrates one embodiment of a distal section of a leadbody of the present invention and its corresponding bending stiffnessprofile;

[0019]FIG. 3B illustrates a conventional lead implanted within a rightatrium;

[0020]FIG. 3C illustrates a lead of the present invention implantedwithin a right atrium;

[0021] FIGS. 4A-4E illustrate various means of increasing the beingstiffness of the distal section of a lead body in the present inventionas a function of axial distance;

[0022]FIG. 5 shows a partial cross-sectional view of a heart having oneembodiment of a lead of the present invention disclosed therein;

[0023] FIGS. 6A-6C illustrate various principles associated with bendingstiffness in respect of several embodiments of the present invention;

[0024] FIGS. 7A-7C illustrate schematically several differentembodiments of leads of the present invention and their correspondingbending stiffnesses and derivatives of stored mechanical energy withrespect to axial distance;

[0025]FIGS. 8A and 8B show two different embodiments of a lead body ofthe present invention in cross-section;

[0026]FIGS. 9A and 9B illustrate combined cross-sectional andperspective views of two different lead bodies of the present invention;

[0027]FIG. 10 illustrates an enlarged cross-sectional view of oneembodiment of a lead of the present invention disposed within portionsof the venous anatomy;

[0028]FIG. 11 illustrates one embodiment of the present inventionadapted for implantation within various portions of the venous anatomy;

[0029]FIG. 12 illustrates several methods of implanting a lead of thepresent invention within a human heart and electrically stimulatingsame.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030]FIG. 1A shows human heart 1 with medical electrical lead 10 of thepresent invention implanted therein. Proximal end 20 of medicalelectrical lead 10 is connected to implantable cardiac stimulator 30 bymeans of s connector or terminal 18. Cardiac stimulator 30 may be apacemaker, an implantable pulse generator (IPG), an implantablecardiodefibrillator (ICD), a pacer-cardioverter-defibrillator (PCD), orany other type of similar cardiac stimulator well known in the art.Medical electrical lead 10 comprises proximal portion 20, distal portion22 and lead body 12. Tip 50 is disposed at the distalmost end of lead10. Electrode 14 may be positioned near tip 50 or at any other suitablelocation along lead body 12. Tip 50 may also have disposed thereon oradjacent thereto tines 57 or any other positive fixation means such as ahelical screw, barb, hook or the like.

[0031] As shown in FIG. 1A, lead 10 of the present invention may beimplanted in right atrium 3, and preferably displays a J-shaped curve atthe distal end thereof upon implantation. Medical electrical lead 10comprises one or more electrodes 14 disposed thereon for pacing, sensingand/or defibrillating heart 1.

[0032] Referring now to FIG. 1 B, there is shown human heart 1 withmedical electrical lead 10 of the present invention implanted withinright ventricle 5. Distal portion 22 of medical electrical lead 10 maysimilarly exhibit a J-shaped curvature similar to that shown in FIG. 1A.In a preferred embodiment of the present invention distal portion 22 ofmedical electrical lead 10 does not have a pre-formed J-shaped curveformed therein, but rather prior to implantation assumes a substantiallystraight configuration which facilitates implantation thereof. In lesspreferred embodiments of the present invention, however, distal portion22 of lead 10 may be pre-shaped as desired into, for example, a J-shape.

[0033] It is a basic principle of the present invention that distalportion 22 of lead body 12 exhibits increased bending stiffness relativeto sections of lead body 12 disposed proximally therefrom. Such abending stiffness profile as a function of axial distance x has thesurprising result of a distally directed force acting upon lead 10 tothereby push lead 10 forwardly or distally, more about which we saybelow.

[0034] Because the bending stiffness of lead 10 increases with axialposition x, the amount of energy stored along the curve formed in distalsection 22 depends on the position of distal section 22 relative to thecurve. When the distal section 22 is moved forward along the curve, thebending stiffness and corresponding stored energy of the portion ofsection 22 disposed in the curve decreases. That is, a lead which priorto implantation assumes a substantially straight shape and in whichdistal portion 22 exhibits variable bending stiffness, where the bendingstiffness increases in the distal direction upon implantation, exerts apositive tip pressure or force on the walls of atrium 3 or ventricle 5when it is shaped into a curved J-shape in an attempt to minimize theamount of stored mechanical potential energy. In fact, when lead 10exhibits a stiffness gradient of about 1 Nmm/radian in distal section22, a tip force of about 0.1 N results (assuming lead 10 has been bentthrough a 180° curve over a 15 mm curve radius).

[0035] In the present invention, it is contemplated that bendingstiffness gradients of distal section 22 of lead 10 range between about0.05 and about 1.0 Nmm per radian, or between about 0.05 and about 1.5Nmm per radian. Forces exerted by tip 50 of lead 10 of the presentinvention may range between about 0.005 N and about 0.1 N. Other rangesof bending stiffness gradients and forces are also contemplated in thepresent invention, such as stiffness gradients ranging between about 0.1to about 0.5 Nmm per radian, and forces exerted by distal section 22ranging between about 0.01 N and about 0.05 N. Other ranges of stiffnessgradients and forces are likewise contemplated in the present inventioneven though not explicitly set forth herein.

[0036] Referring now to FIG. 2A, the feasibility of the presentinvention was tested by means of a computer program. The physicalparameters employed in the program are shown in FIG. 2A. The resultsprovided by the program are shown in FIG. 2B.

[0037] The lead design parameters employed as inputs to the programincluded the following: lead 10, rigidly clamped by the “fixed world” 99at its proximal end, comprised an originally straight lead section 22which was bent over about 180°. Tip 50 was assumed to form a sectionabout 15 mm long, while reinforced or relatively stiff section 2 had itslength varied between about 10 mm and 40 mm. Reinforced section 2corresponds to relatively stiff section described here below inconnection with various embodiments of the present invention. Section 4of lead body 12 shown in FIG. 2A corresponds to relatively flexiblesection 4 described below in connection with various embodiments of thepresent invention. The calculated tip force exerted on distal section 22of lead 10 having various different lengths of relatively stiff section2 are shown in FIG. 2B. The calculated tip forces shown in FIG. 2Bproved the feasibility of the basic concept of the present invention. Apositive force component F_(y) of the total force F acting on tip 50 isrepresentative for a stable position of tip 50. For a reinforcement (2)of 40 mm length (E40), F_(y) is positive for vertical tip positions fromY=−10 mm till y=30 mm, corresponding to a range of 40 mm.

[0038] Referring now to FIG. 3A, there is shown lead 10 of the presentinvention having disposed immediately therebelow its correspondingbending stiffness (S_(b)) profile, where the bending stiffness varies asa function of axial distance x. Lead 10 comprises distal portion 22,lead body 12, relatively flexible section 4, relatively stiff section 2,tip 50 and tines 57. Other positive fixation means such as a helicalscrew, barb, hook, and the like may be disposed on or near tip 50, suchas optional tip 50 illustrated to the right of tined tip 57 in FIG. 3A.The bending stiffness profile of lead 10 is shown to increase over thelength of that portion of lead 10 which is to have a J-shaped curve uponimplantation within right atrium 3 (e.g., at least portions of distalsection 22). That is, when initially straight lead 10 is implanted inhuman heart 1, lead 10 is bent into a J-shaped configuration withinright atrium 3 (or right ventricle 5).

[0039] Referring now to FIGS. 3B and 3C, there are shown two differentleads. FIG. 3B shows conventional lead 10 disposed in right atrium 3such that distal portion 22 is bent into a J-shape. Superimposed uponthe cross-section of lead 10 and heart 1 in FIG. 3B are correspondingforce vectors acting upon distal portion 22 of lead 10 in response tothe axial forces exerted by lead 10 upon the walls of atrium 3. Notethat those force vectors are relatively uniform respecting magnitude.

[0040] Contrariwise, the force vectors shown in FIG. 3C acting upon lead10 are not uniform, and increase in magnitude in the distal direction oflead 10. This feature of the present invention results in the distallydirected forward pushing force (F_(push)) shown in FIG. 3C which isconteracted by the axial tip force F_(tip) and an axial force on leadbody 12, to provide a static equilibrium of forces and moments. Asdiscussed hereinabove, that pushing force is the direct result of theunique bending stiffness profile of distal section 22 of lead body 12.

[0041] FIGS. 4A-4E illustrate various means of increasing the bendingstiffness of distal section 22 of lead body 10 as a function of axialdistance x. FIG. 4A shows coil 59 disposed within lead body 12. Thepitch of spring of coil 59 increases in the axial direction x to therebyincrease the bending stiffness as one moves towards tip 50 along leadbody 12.

[0042]FIG. 4B shows another embodiment of the present invention, wherethe bending stiffness of lead body 12 increases in the distal directionalong axial direction x by means of increasing the thickness of theouter covering or layer of lead body 12. Note that an inwardly disposedlayer or substrate could also exhibit increasing thickness in the distaldirection to achieve the same result. Likewise, a material of uniformthickness but exhibiting changes in its elastic modulii as a function ofaxial distance x could also be employed to achieve the same result.

[0043]FIG. 4C shows another embodiment of the present invention, wherean increase in bending stiffness with increasing axial distance x isachieved by increasing the diameter of lead body 12 in the distaldirection.

[0044]FIG. 4D shows distal portion 22 of lead 10 having successivelymore layers 61A, 61 B and 62B, disposed over outer portions thereof toimpart an increase in bending stiffnessess as a function of axialdistance x.

[0045]FIG. 4E shows one embodiment of the present invention where anincrease in bending stiffness with axial distance x is achieved bydecreasing the diameter of a coil disposed therein as a function ofaxial distance x.

[0046] It will now become apparent to those skilled in the art, afterhaving read the specification and reviewed the drawings thereof, thatmany other means of achieving the results of the present invention arepossible, where the bending stiffness of lead body 12 in distal section22 increases in the distal direction.

[0047] It is contemplated in the present invention that means of varyingthe bending stiffness of the distal section of a lead of the presentinvention other than those described here and above respective FIGS.4A-4E fall within the scope of the present invention. For example, thematerial from which lead body 12 is formed may be varied compositionallyor otherwise as a function of axial distance x, to thereby effectuatechanges in the bending stiffness thereof. The degree to which a polymerforming lead body 12 is cross-linked may be varied as a function ofaxial distance x. The density of the polymers or other materialsemployed to form lead body 12 may be varied as a function of axialdistance x. The molecular weight of the polymers or other materials fromwhich lead body 12 is formed may be varied as a function of axialdistance x. A flexible tubular member containing a shape-memory tube maybe included in a lumen extending along a central axis of the lead body,and a control system may then selectively heat portions of theshape-memory tube to change the bending stiffness or shape thereof. Theforegoing and other methods of varying the bending stiffness of thedistal section of a lead body of the present invention are contemplatedin the present invention. See, for example, U.S. Pat. No. 5,437,632 for“Variable stiffness balloon catheter”; U.S. Pat. No. 5,499,973 for“Variable stiffness balloon dilatation catheters”; U.S. Pat. No.5,531,685 for “Steerable variable stiffness device”; U.S. Pat. No.5,639,276 for “Device for use in right ventricular placement and methodfor using same”; U.S. Pat. No. 5,833,604 for “Variable stiffnesselectrophysiology catheter”; and U.S. Pat. No. 5,733,496 for “Electronbeam irradiation of catheters to enhance stiffness”, the disclosures ofwhich are hereby incorporated by reference herein, each in itsrespective entirety.

[0048] Referring now to FIG. 5, there is shown a cross-sectional view oflead 10 disposed in, for example, posterior cardiac vein 17 of heart 1via coronary sinus 13 and great cardiac vein 23. At least portions ofdistal portion 22 of lead 10 are located in posterior cardiac vein 17.FIG. 5 illustrates how lead 10 must be routed through a series ofwinding tortuous pathways when implanted in the cardiac veins. Suchpathways not only make implantation and placement of lead 10 in desiredportions of heart 1 difficult, but also have a tendency to cause priorart leads to be pushed out of the cardiac vein in which they have beenlocated by implantation, further discussion concerning which followsbelow.

[0049] Continuing to refer to FIG. 5, there is shown medical electricallead 10 of the present invention, which prior to implantation mostpreferably has a straight distal section 22 and which is adapted forimplantation within coronary sinus 13, great cardiac vein 23, or withinany other of the left ventricular coronary veins or left atrial veinswhen appropriately configured and dimensioned. In the present invention,the bending stiffness of distal section 22 of lead 10 is made variableso as to increase or decrease in a predetermined singular or periodicfashion.

[0050] Thus, in one embodiment of the present invention distal portion22 of lead 10 has at least one distalmost stiff section 2 disposeddistally of a flexible section 4 located adjacent thereto. That is, leadbody 12 may be configured to have at least one stiff section 2 and atleast one flexible section 4 located in distal portion 22 thereof.Medical electrical lead 10 of the present invention may additionallyhave adjacent adjoining portions which alternate between being flexibleand being stiff relative to one another. More particularly, theflexibility or stiffness of sections 2 and 4 of lead 10 may be moreaccurately characterized as having different bending stiffnesses(S_(b)), wherein the ratio of the bending stiffness of the stiff section2 (S_(bs)) is at least 1.5 times that of the bending stiffness of theflexible section 4 (S_(bf)). The bending stiffness ratios between moreflexible sections 4 and more stiff sections 2 of lead may also exceedabout 1.8, about 2, about 2.2, about 2.4, about 2.6, about 2.8, about3.0, about 4, about 5, about 6, about 7, about 8, about 9, about 10,about 20, about 30, about 40, about 50, about 100 or even greater.

[0051] Expressed mathematically, the ratio of bending stiffnesses ofstiff sections 2 and flexible sections 4 of lead 10 of the presentinvention are: $\begin{matrix}{1.5 \leq \frac{S_{bs}}{S_{bf}} \leq 100} & \left( {{eq}.\quad 1} \right) \\{1.5 \leq \frac{S_{bs}}{S_{bf}} \leq 20} & \left( {{eq}.\quad 2} \right) \\{2 \leq \frac{S_{bs}}{\quad S_{bf}} \leq 10} & \left( {{eq}.\quad 3} \right) \\{2 \leq \frac{S_{bs}}{S_{bf}} \leq 5} & \left( {{eq}.\quad 4} \right)\end{matrix}$

[0052] When lead 10 is advanced through coronary sinus 13 into greatcardiac vein 23 and then into posterior cardiac vein 17, for example,lead 10 will assume a winding, almost wave-shaped configuration, suchthat distal portion 22 is curved at the transition between coronarysinus 13 and posterior cardiac vein 17 as well as along the pathway ofposterior cardiac vein 17.

[0053] It has been discovered that lead 10 will attempt to assume aposition with minimal stored mechanical energy after having beenimplanted within veins 17 and 13. It has further been discovered thatflexible sections 4 of lead 10 are most preferably located in thoseportions of the venous pathway having the curves of smallest radius (andtherefore requiring the lowest amounts of stored potential mechanicalenergy).

[0054] Thus, first radius of lead body curvature 36 shown in FIG. 1A and1B is most preferably located along those portions of lead 10 whichcomprise flexible portion 4 of lead body 12. Likewise, second, third andfourth radii of lead body curvatures 38, 40 and 42, respectively, shownin FIG. 5 are likewise located along portions of lead 10 comprisingflexible portions 4. Relatively straight portions of lead 10, implantedwithin human heart 1 in a desired position preferably compriserelatively stiff portions 2 of lead body 12 as shown in FIG. 5.

[0055] In the present invention, therefore, moving flexible sections 4from their locations within first, second, third and fourth curves 38,40 and 42 requires that an axial force be exerted on lead 10 to advancelead 10 distally (i.e., exertion of a pushing force) or to retract lead10 (i.e., exertion of a pulling force). Thus, owing to the uniquevariation of bending stiffness along the length and axial direction x oflead body 12 of lead 10, lead 10, once implanted, has a pronouncedtendency to remain implanted and not to become dislodged from thecardiac vein within which it has been implanted.

[0056] FIGS. 6A-6C illustrate various principles associated with theforegoing discussion concerning FIGS. 1 and 5. The principle of arelatively straight lead having variable bending stiffness as a functionof lead position is based on two mechanical laws: (1) a mechanical bodysubjected to an external load or deformation assumes a shape whichminimizes the potential mechanical energy stored in that body; and (2)variation of the stored potential energy in a body with displacement ofthe body results from an external force acting thereon. The externalforce (F) equals the derivative of energy (E) with respect todisplacement (x) as shown below: $\begin{matrix}{F = \frac{E}{x}} & \left( {{eq}.\quad 5} \right) \\{\frac{E}{x} = {{\frac{\phi_{b} \cdot R_{b}}{R_{b}} \cdot \frac{S_{b}}{x}} = {\phi_{b} \cdot \frac{S_{b}}{x}}}} & \left( {{eq}.\quad 6} \right)\end{matrix}$

[0057] where S_(b)=bending stiffness, R_(b)=bending radius and φ_(b) isthe bend angle.

[0058] In FIG. 6A the additional energy stored in curved flexiblesection 4 of lead body 12 is defined by the force F required to displacelead 12 into the position shown along with the change in displacementdX. FIGS. 6B and 6C illustrate that the amount of bending energyrequired to bend lead body 12 through an approximate 90° curvature isgreater for the geometry shown in FIG. 6C than is that illustrated inFIG. 6B. This is because stiff section 2 is located in the curvedsection of lead body 12 is FIG. 6C. Greater bending energy is thereforerequired to bend lead body 12 into the configuration shown in FIG. 6Cthan the configuration shown in FIG. 6B, where flexible section 4 isdisposed along most of the curved section. In other words, the leadconfiguration shown in FIG. 6B is mechanically more stable than is theconfiguration shown in FIG. 6C because the configuration of FIG. 6Cachieves a lower stored mechanical energy level.

[0059] Applying the law of minimum stored mechanical energy to thedistal section of lead 10, we can draw the following conclusions. Whenlead 10 is implanted in coronary sinus 13 and great cardiac vein 23,mechanical energy is stored in those curved sections of lead 10 whichare located in the transition from coronary sinus 13 to coronary vein 23or 17. Such stored mechanical energy is proportional to the stiffness oflead 10 and the length being curved, as well as to the curvature (whichis the inverse of the bending radius). Assume that the curvature isdetermined mainly by the venous anatomy, that the angle or curvature isabout 90° and that the bend radius is about 5 mm. Such a curve will bemaintained by forces acting on both sides of the lead body. The energystored in lead body 12 is proportional to the average stiffness in thecurved section.

[0060] Because the stiffness in the curved section varies with theposition of the lead along the curve, the average stiffness of the leadbody disposed in the curve will change if the lead is moved along thecurve or the curve is moved with respect to the lead. Thus, axialdisplacement x of lead 10 along the curve defined by the venous anatomyresults in a change in stored mechanical energy. If a lead of thepresent invention has been implanted within the venous anatomy of apatient properly, additional energy from an external source (e.g., aphysician pulling or pushing the lead along the axial direction x) willhave to be provided to displace lead 10 from its preferred minimumstored mechanical energy position.

[0061] It has been discovered that it is preferred to locate the mostflexible section of the lead in those portions of the venous anatomywhich exhibit the greatest curvature (or maximum bend radii). In such aconfiguration, the stiffness of lead 10 increases both proximally anddistally with respect to the flexible section disposed in the curvedsection, and thus the stored energy of the lead body will become greaterif the lead is moved either distally or proximally, or the venousanatomy moves with respect to the lead either distally or proximally.Stored mechanical energy is maintained at a minimum when the flexiblesection remains in the center of the curve. This results in a stablemechanical equilibrium, which in turn requires that external force ofsufficient magnitude be exerted on lead 10 to move it distally orproximally from its minimum stored mechanical energy position.

[0062] In accordance with some embodiments of the present invention,lead 10 may be configured to have one relatively stiff portion 2adjoining a relatively flexible portion 4, or may have a series ofalternating relatively stiff portions 2 and relatively flexible sections4. The bending stiffness of adjoining sections may increase or decreasein step-wise fashion, or may increase or decrease monotonically,exponentially or logrithmically. The respective lengths of relativelystiff portions 2 and relatively flexible portions 4 may also be variedaccording to the particular venous anatomy in which lead 10 is to beimplanted.

[0063] In one embodiment of the present invention lead 10 issubstantially straight prior to implantation and exhibits variablestiffness in distal portion 22 thereof such that at least one flexiblesection 4 adjoins proximally disposed and adjacent stiff portion 2 anddistally disposed and adjacent stiff section 2, respectively. Such alead configuration exhibits a bilateral, stable equilibrium (see FIG.7C).

[0064] In another embodiment of the present invention lead 10 has asingle stiff section 2 disposed in distal portion 22 which has arelatively flexible section 4 disposed proximally therefrom and adjacentthereto. Such a lead configuration has a unilateral, mechanically stableequilibrium, wherein the bending stiffness junction between sections 2and 4 of differing stiffness is optimally placed at either end of acurve in a venous anatomy (see FIG. 7B).

[0065] FIGS. 7A-7C illustrate the behavior of several selectedembodiments of lead 10 of the present invention, where bending stiffness(S_(b)) of lead body 12 is varied as a function of lead axial positionx. In each of FIGS. 7A-7C, the upper diagram illustrates bendingstiffness S_(b) as a function of lead axial position x, the middlediagram illustrates the derivative of stored mechanical energy E withrespect to axial distance x, (such derivative of stored mechanicalenergy being proportional to the axial force F_(ax) exerted by thelead), and the lower diagram illustrates a lead structure correspondingto the bending stiffnesses and axial forces illustrated thereabove. Inall of FIGS. 7A-7C the distal tip of the lead is positioned at the rightside of the diagrams, relatively stiff portions of lead 10 are indicatedby numeral 2 and relatively flexible sections of lead 10 are indicatedby numeral 4.

[0066] Referring now to FIG. 7A, the monotonic increase in bendingstiffness begins at the junction between sections 4 and 2 and increasesto a maximum at tip 50. Such a configuration results in an axial force(F_(ax)) being exerted by lead 10 as shown in the middle diagram. Here,as in other axial force diagrams which follow below, a positive axialforce is one which acts to pull the lead in a distal direction, whereasa negative axial force acts to pull a lead in a proximal direction(i.e., out of the vein within which it has been implanted).

[0067] Referring now to FIG. 7B, there is shown a lead exhibiting astep-wise jump in bending stiffness which occurs at the junction betweensections 2 and 4 thereof. Once distalmost stiff portion 2 has beenpushed beyond the venous curve of interest, and flexible section 4 isdisposed in such curve, the axial force (F_(ax)) exerted by distalportion 22 of lead 10 upon the venous anatomy is again positive andtends to retain the lead in the implanted position unless an axialpulling force operating in the proximal direction is exerted on lead 10to pull lead 10 around the curve of interest to thereby overcome F_(ax).

[0068]FIG. 7C shows lead 10 having a series of contiguous alternatingrelatively flexible and relatively stiff sections 2 and 4, respectively.Lead 10 shown in FIG. 7C exhibits a number of points of bilateralstability separated by a distance equal to the length of relativelyflexible and relatively stiff sections 4 and 2, respectively. Such alead configuration has the advantage that a tip or electrode thereof maybe placed at any of several positions along one or more coronary veins.That is, the embodiment of lead 10 shown in FIG. 7C has a number ofdifferent minimum mechanical energy storage positions which it mayassume within the venous anatomy of a patient. The relative lengths ofrelatively flexible portions 4 and relatively stiff portions 2 may bevaried according to the radii of the different venous curves which areanticipated to be encountered during lead implantation.

[0069] Thus, if it is anticipated that lead 10 will be implanted in aportion of the venous anatomy which is characterized by tightly curvedvenous portions, lead 10 may be configured to have relatively shortstiff and flexible sections 2 and 4, respectively, to provide optimalresults. Contrariwise, in the event the venous anatomy to be encounteredduring the implantation process is expected to be characterized byrelatively gently curves, lead 10 may be configured such that relativelystiff sections 2 and relatively flexible section 4 have longer lengthsto thereby provide optimal results. Lead 10 may also be appropriatelyconfigured such that portions 2 and 4 are of appropriate differinglengths for small, medium, and large radii curves encountered by thesame lead 10.

[0070] It is important to note that when relatively stiff portion 2 oflead 10 is disposed in or along a curved section of the venous anatomy,an unstable mechanical equilibrium associated with a local maximum ofstored potential mechanical energy being disposed in the curve results.It is therefore desired in the present invention that lead 10 havealternating relatively flexible sections 4 and relatively stiff sections2 located within the venous anatomy in such a way that relativelyflexible sections 4 are located in at least the major curves thereof.

[0071] The principle of varying the bending stiffness of lead 10 as afunction of axial distance x may also be expanded to cover circumstanceswhere the bending stiffness (S_(b)) is symmetric and equal around eachaxis of bending, or asymmetric and unequal around each axis of bending.

[0072] Referring now to FIGS. 8A and 8B, there are shown incross-section lead body 12 exhibiting symmetric equal bendingstiffnesses around each axis of bending in FIG. 8A and lead body 12having asymmetric unequal bending stiffnesses around each axis ofbending in FIG. 8B. Thus, lead 10 shown in FIG. 8A may be bent in anydirection from 0° to 360° without any change in bending moment beingrequired. Contrariwise, lead 10 shown in FIG. 8B requires more bendingmoment when lead 10 is bent in the directions of 0° and 180°, while lessbending moment is required when lead 10 is bent in the 90° and 270°directions.

[0073]FIGS. 9A and 9B illustrate lead bodies which require asymmetricbending moments as a function of angular direction. In order to maintainminimal mechanical energy, lead body 12 illustrated in FIG. 9B willattempt to orient itself along the plane of the curve within which it isdisposed such that bending preferentially occurs over the lead axisalong the most flexible lead cross-section (e.g., the 90° and 270°orientations). This characteristic may be exploited so that lead body 12may be oriented such that an electrode disposed along or near such asection exhibiting asymmetric bending stiffness is strategically placedwithin a vein. Thus, for example, a pacing or defibrillation electrode14 disposed near such an asymmetric bending stiffness section may beoriented towards the myocardium (which may be beneficial in obtaininglow pacing thresholds and improved sensing of signals).

[0074]FIG. 9A illustrates the natural orientation which the lead of FIG.8B will assume within a curved portion of the venous anatomy. The leadconfiguration shown in FIG. 9B is one which requires maximum mechanicalenergy and therefore will not be assumed by lead 10 when disposed in acurved section of the venous anatomy.

[0075] Referring now to FIG. 10, there is shown an enlargedcross-sectional view of lead 10 disposed within posterior cardiac vein17 after having been routed through coronary sinus 13. FIG. 10 shows howvenous vasculature exhibits curves having radii which alternate indirection and magnitude. Bending of lead body 12 along posterior cardiacvein 17 occurs substantially within a single plane (i.e. R₁, R₂ and R₃are disposed substantially in the same plane). Because radii R₁, R₂ andR₃ are so much smaller than radius R₄, more radical bending of lead 10is required in posterior cardiac vein 17. Bending of lead body 12occurring along R₄ of coronary sinus 13 occurs in a plane which isapproximately perpendicular to the plane along which R₁, R₂ and R₃ aredisposed. Note that R₄ is substantially longer than R₁-R₃ and thus thecurve of coronary sinus 13 is not only along a different plane but ofsubstantially less magnitude. Consequently, a preferential orientationof lead 10 is determined principally by radii R₁-R₃ rather than byradius R₄. This, in turn, means that a lead having an asymmetriccross-sectional configuration or bending stiffness which variesasymmetrically as a function of cross-sectional angular position may besuccessfully employed to ensure the retention of lead 10 within adesired portion of the venous anatomy. For example, lead 10 may beconfigured to have a first asymmetric cross-sectional configuration forimplantation along the distalmost portions of a selected cardiac vein ina first preferred orientation where bending radii are small, and to havea second asymmetric cross-sectional configuration for implantation in oralong more proximally disposed portions of the venous anatomy and in asecond preferred orientation, wherein the first and second orientationsare different owing, for example, to the first and second cross-sectionsbeing angularly rotated in respect of one another.

[0076] Assuming the embodiment of the present invention illustrated inFIGS. 8B and 9A is employed for implantation within a desired portion ofthe venous anatomy, such a lead will have two orientations where storedmechanical energy will be achieved, namely at φ=90° or φ=270°, assumingthat the bending stiffness of the lead is equal in those oppositedirections. Electrode 14(b) may be positioned on one side or the otherof lead body 12 to stimulate a desired portion of the heart as shown inFIG. 10. Such positioning may be confirmed through the use of x-ray orecho identification of the orientation of electrode 14(b). If required,lead 10 may be rotated through 180° such that electrode 14(b) faces adesired direction.

[0077]FIG. 11 illustrates another embodiment of the present invention,where lead 10 is adapted for implantation within right atrium 3,coronary sinus 13 and a selected cardiac vein. The distal tip 50 of lead10 is disposed in the selected cardiac vein, while proximal therefrom aportion of lead 10 having bending stiffness characteristics which differfrom those of the distalmost portions of lead 10. More particularly, andreferring now to FIG. 11 again, it will be seen that distal portion 22of lead 10 is characterized in having a bending stiffness profile whichalternates between relatively stiff portions 2 and relatively flexibleportions 4. Proximal from such sections of alternating relatively stiffand relatively flexible sections 2 and 4 there is disposed a section oflead body 12 in which bending stiffness increases in the distaldirection, most preferably in the manner shown in FIG. 11. Note,however, that the increase in bending stiffness shown over thoseportions of lead 10 illustrated in FIG. 11 intended for implantation inright atrium 3, and optionally at least portions of coronary sinus 13,may increase monotonically, exponentially, step-wise or logrithmically.The important point is that bending stiffness over the portion of thelead implanted within the right atrium and optionally at least portionsof the Coronary sinus have an increasing bending stiffness to create aforce which will have a tendency to push the lead in the distaldirection, even after implantation.

[0078] An outer layer or sleeve may surround lead body 12. Without anylimitation intended, the sleeve may be constructed from a carbon coatedsilicone, steroid, steroid-eluting silicone, or a combination ofsilicone and an anti-fibrotic surface treatment element. Any of thosecompositions may help reduce tissue response to lead insertion so thatlead 10 will not cause clots or adhere to the vessel wall, therebyallowing retraction of the lead in the future, if necessary. Thesecompositions may also help prevent encapsulation of the electrode,thereby enhancing the effectiveness of the pacing and sensingcapabilities.

[0079] One or more electrical conductors are disposed on or in lead body12 and convey signals sensed by electrode 14 or permit the delivery ofelectrical pacing or defibrillation signals therethrough. Suchconductors may be helically wound coils or multistrand twisted cables,ETFE coated, or fixed within a longitudinally disposed lumen of the leadbody 12. The distal end of lead conductor 16 may be attached toelectrode 14 while the proximal end thereof is attached to terminal pin18 by crimping or laser weld means well known to those skilled in theart. Without any limitation intended, electrode 14 and terminal pin 18may be manufactured from titanium or platinum-plated titanium. Conductor16 preferably comprises electrically conductive braided or strandedwires. A lumen 30 may be formed within lead body 12 wherein a stylet ofknown construction may be positioned therein.

[0080] Having generally explained the features and positioning of lead10, and referring now to the flow diagram of FIG. 12, some methods ofpacing and/or defibrillating a patient's heart using a coronary veinlead 10 and implanting same will now be discussed. The method of pacinga patient's heart identified in the flow chart of FIG. 12 allows a userto effectively pace the left ventricle without increased risk of anischemic episode.

[0081] The operator first positions a guide catheter of the tear awaytype known to those skilled in the art within coronary sinus 13 (block150). Although the use of a guide catheter is not absolutely necessary,a guide catheter increases the ability of the operator to properlyposition lead 10 within a preselected coronary vein. Once the guidecatheter has been positioned within coronary sinus 13, lead 10 isinserted through the lumen of the guide catheter and into apredetermined coronary vein under fluoroscopic observation (see Block152). Lead 10 is positioned within the selected coronary vein, whereinthe electrodes of lead 10 are aligned with the selected chambers to bepaced. Those skilled in the art will appreciate that the electrodes maybe constructed from a radiopaque material such that the position of theelectrode is readily determined. After lead 10 is appropriatelypositioned in heart 1, the stylet or guide wire (if present) is removedfrom lead 10 (see block 154). The catheter is then removed from coronarysinus 13 (block 156) and the catheter is torn away as the catheter ispulled past the terminal pins of lead 10. Before removing the catheterfrom lead, however, electrical measurements may be taken. As notedabove, a guide catheter may be used to direct a guide wire which is usedto guide a support catheter to a desired position within a pre-selectedcoronary vein. The support catheter is then used to position lead 10 asdescribed above.

[0082] After the guide catheter has been removed, the operator decideswhether there are additional coronary vein leads to be inserted andpositioned within the coronary veins of a patient's heart (see decisionblock 158). If other leads 10 are to be positioned within pre-selectedcoronary veins, then the above steps represented by blocks 150-156 arerepeated (see loop 160). Those skilled in the art will appreciate thatan additional lead of suitable construction could be positioned withinthe right atrium or ventricle. If no other leads 10 are to be insertedand positioned, then terminal pins 18 attached to each coronary veinlead 10 are coupled to corresponding terminal ports of cardiacstimulator 30 (block 162). Stimulator 30 is then programmed by knownmeans to transmit a pacing and/or defibrillation pulse through eachcoupled lead 10 (block 164) to pace or defibrillate the pre-selectedchamber of the patient's heart.

[0083] For placement of the lead tip in the atrial appendix a stylet isinserted and the lead pushed until the distal end of the lead is in theright atrium. The stylet is replaced with a J-shaped stylet to impartcurvature on the distal end of the lead and to place the tip in thedesired location of the right atrial appendix.

[0084] Once lead 10 of a suitable embodiment of the present inventionhas been inserted and positioned in heart 1, and without any limitationintended, the operator has the ability to, for example, pace or senseboth the left atrium and left ventricle, or pace or sense the leftatrium, left ventricle, and right atrium. When a separate rightventricular lead is positioned, pacing and/or sensing from all chambersof the heart may be possible. The diameter and construction of lead 10provides the flexibility necessary to reduce substantially thelikelihood that flexure of lead 10 will result in the coronary veinbeing eroded through. In this regard, the lead body 12 of lead 10 may becoated or impregnated with a biomedical steroid to reduce theinflammatory response of the coronary veins to the insertion andpositioning of lead 10 therein. The selected biomedical steroid may alsobe used to reduce the amount of fiber build-up occurring between lead 10and the coronary vein. Lead 10 may also be constructed to include ananchoring member such that lead 10 may be additionally anchored withinthe coronary vein or Coronary sinus.

[0085] Although specific embodiments of the invention have been setforth herein in some detail, it is to be understood that this has beendone for the purposes of illustration only, and is not to be taken as alimitation on the scope of the invention as defined in the appendedclaims. Thus, the present invention may be carried out by usingequipment and devices other than those described specifically herein.Various modifications, both as to the equipment and operatingprocedures, may be accomplished without departing from the scope of theinvention itself. It is to be understood that various alternatives,substitutions and modifications may be made to the embodiment describeherein without departing from the spirit and scope of the appendedclaims.

[0086] In the claims, means-plus-function clauses are intended to coverthe structures described herein as performing the recited function andnot only structural equivalents but also equivalent structures. Thus,although surgical glue and a screw may not be structurally similar inthat surgical glue employs chemical bonds to fasten biocompatiblecomponents together, whereas a screw employs a helical surface, in theenvironment of fastening means, surgical glue and a screw are equivalentstructures.

[0087] All patents cited hereinabove are hereby incorporated byreference into the specification hereof, each in its respectiveentirety.

I claim:
 1. An elongated implantable medical electrical lead forelectrically stimulating a human heart or sensing electrical signalsoriginating therefrom, comprising: (a) a lead body having proximal anddistal sections; (b) at least one electrode for sensing or electricallystimulating the heart; (c) a proximal end comprising an electricalconnector, the electrical connector being contiguous with the proximalsection of the lead body; (d) a distal end contiguous with the distalsection of the lead body; (e) at least one electrical conductor havingproximal and distal ends, the distal end of the conductor beingoperatively connected to the at least one electrode, the proximal end ofthe conductor being operatively connected to the electrical connector;wherein the distal section of the lead body comprises at least first andsecond segments, the first segment having a bending stiffness S_(bs)which exceeds the bending stiffness S_(bf) of the second segment, thefirst and second segments being configured and dimensioned to impart adistally directed force to the distal end of the lead when the first andsecond segments are subjected to a bending moment resulting in asufficient curvature of the distal section of the lead body.
 2. Themedical electrical lead of claim 1, wherein the ratio of the bendingstiffness of the first segment (S_(bs)) in respect of the second segment(S_(bf)) is defined by the equation:$1.5 \leq \frac{S_{bs}}{S_{bf}} \leq 100$


3. The medical electrical lead of claim 1, wherein the ratio of thebending stiffness of the first segment (S_(bs)) in respect of the secondsegment (S_(bf)) is defined by the equation:$1.5 \leq \frac{S_{bs}}{S_{bf}} \leq 20$


4. The medical electrical lead of claim 1, wherein the ratio of thebending stiffness of the first segment (S_(bs)) in respect of the secondsegment (S_(bf)) is defined by the equation:$1.5 \leq \frac{S_{bs}}{S_{bf}} \leq 10$


5. The medical electrical lead of claim 1, wherein the ratio of thebending stiffness of the first segment (S_(bs)) in respect of the secondsegment (S_(bf)) is defined by the equation:$2 \leq \frac{S_{bs}}{S_{bf}} \leq 6$


6. The medical electrical lead of claim 1, wherein the bending stiffnessof the first segment (S_(bs)) is at least 1.5 times that of the bendingstiffness of the second segment (S_(bf)).
 7. The medical electrical leadof claim 1, wherein the bending stiffness of the first segment (S_(bs))is at least 1.8 times that of the bending stiffness of the secondsegment (S_(bf)).
 8. The medical electrical lead of claim 1, wherein thebending stiffness of the first segment (S_(bs)) is at least about 2times that of the bending stiffness of the second segment (S_(bf)). 9.The medical electrical lead of claim 1, wherein the bending stiffness ofthe first segment (S_(bs)) is at least about 4 times that of the bendingstiffness of the second segment (S_(bf)).
 10. The medical electricallead of claim 1, wherein the bending stiffness of the first segment(S_(bs)) is at least about 6 times that of the bending stiffness of thesecond segment (S_(bf)).
 11. The medical electrical lead of claim 1,wherein the ratio of the bending stiffness of the first segment (S_(bs))in respect of the second segment (S_(bf)) is selected from the groupconsisting of at least about 2.2, at least about 2.4, at least about2.6, at least about 2.8, at least about 3.0, at least about 4, at leastabout 5, at least about 6, at least about 7, at least about 8, at leastabout 9, at least about 10, at least about 20, at least about 30, atleast about 40, at least about 50, and at least about
 100. 12. Themedical electrical lead of claim 1, wherein the distal section of thelead body comprises a plurality of alternating series of substantiallyadjoining first and second segments.
 13. The medical electrical lead ofclaim 1, wherein the distal section of the lead body comprises a thirdsegment having a bending stiffness which exceeds the bending stiffnessof the second segment, the second segment being disposed between thefirst and third segments.
 14. The medical electrical lead of claim 1,wherein the distal section of the lead body comprises a third segmenthaving a bending stiffness which is less than the bending stiffness ofthe first segment, the first segment being disposed between the secondand third segments.
 15. The medical electrical lead of claim 1, whereinthe bending stiffness of the distal section of the lead body increasesdistally in one of step-wise, monotonic, exponential or logarithmicfashion.
 16. The medical electrical lead of claim 1, wherein the bendingstiffness of the distal section of the lead body decreases distally inone of step-wise, monotonic, exponential or logarithmic fashion.
 17. Themedical electrical lead of claim 1, wherein the lengths of the first andsecond segments are selected according to a particular venous anatomy inwhich the lead is to be implanted.
 18. The medical electrical lead ofclaim 1, wherein the lead assumes a substantially straight shape priorto implantation.
 19. The medical electrical lead of claim 1, wherein thelead body has at least one pre-formed curve disposed therein.
 20. Themedical electrical lead of claim 1, wherein the distal section of thelead body is formed into a curved configuration.
 21. The medicalelectrical lead of claim 1, wherein the distal section of the lead bodyand the first and second sections thereof are dimensioned and configuredfor use in a coronary sinus or cardiac vein of the heart.
 22. Themedical electrical lead of claim 1, wherein a fixation device isattached to the lead body.
 23. The medical electrical lead of claim 22,wherein the fixation device is selected from the group consisting of ahelical screw, a barb, a hook, at least one tine, and at least one arm.24. The medical electrical lead of claim 22, wherein the fixation deviceis disposed near the distal end.
 25. The medical electrical lead ofclaim 1, wherein the at least one electrode is located along the distalsection of the lead body at a location appropriate to locate theelectrode adjacent the left atrium or left ventricle of the heart. 26.The medical electrical lead of claim 1, wherein the lead, the at leastone electrode, and the first and second segments are configured anddimensioned to form a single pass lead for dual chamber pacing of a leftatrium and a left ventricle via implantation within a coronary sinus anda great cardiac vein of the heart.
 27. The medical electrical lead ofclaim 1, wherein the lead body is configured to permit preferentialbending thereof along at least one predetermined bending plane.
 28. Themedical electrical lead of claim 1, wherein the lead body is configuredto permit three dimensional bending thereof along at least twopredeteremined ending planes.
 29. The medical electrical lead of claim1, wherein the bending stiffness of at least one of the first segmentand the second segment is rotationally symmetric.
 30. The medicalelectrical lead of claim 1, wherein the bending stiffness of at leastone of the first segment and the second segment is rotationallyasymmetric.
 31. The medical electrical lead of claim 30, wherein the atleast one electrode and the lead body are dimensioned and configuredsuch that when the lead is appropriately implanted within a venousportion of the heart the rotationally asymmetric segment may be employedby a physician to orient placement of the at least one electrode suchthat the electrode is pressed against or directed towards a selectedportion of the heart.
 32. The medical electrical lead of claim 1,wherein the lead has a unipolar electrode configuration.
 33. The medicalelectrical lead of claim 1, wherein the lead has a multipolar electrodeconfiguration.
 34. The medical electrical lead of claim 1, wherein thelead further comprises at least one defibrillation electrode.
 35. Themedical electrical lead of claim 34, wherein the at least onedefibrillation electrode is one of a coil electrode and a ringelectrode.
 36. The medical electrical lead of claim 1, wherein the leadcomprises pacing and defibrillation electrodes.
 37. The medicalelectrical lead of claim 1, wherein the lead body is configured anddimensioned such that when the lead is implanted within a venous portionof the human heart the second segment is located in portions of thevenous portion which exhibit the greatest curvature.
 38. The medicalelectrical lead of claim 37, wherein the bending stiffness of the leadbody increases both proximally and distally in respect of the secondsegment.
 39. The medical electrical lead of claim 1, wherein the secondsegment is disposed proximally from the first segment, the first andsecond segments are contiguous with one another along a junction, andthe junction is located along the lead body at an axial position suchthat when the lead is implanted within a venous anatomy of the humanheart the junction is located near an end of a curve in the venousanatomy.
 40. The medical electrical lead of claim 1, wherein the leadbody comprises a first asymmetric cross-section configured forimplantation in a first preferred orientation in pre-determineddistal-most portions of the heart's venous anatomy where bending radiiare small, a second asymmetric cross-section configured for implantationin a second preferred orientation different from the first orientationin pre-determined portions of the heart's venous anatomy locatedproximal from the distal-most portions thereof.
 41. The medicalelectrical lead of claim 1, wherein the bending stiffness of a proximalportion of the distal section of the lead body increases distally andwherein the proximal portion of the distal section of the lead body isconfigured and dimensioned such that the proximal portion of the distalsection of the lead body is located in a right atrium and a coronarysinus of the heart upon implantation.
 42. The medical electrical lead ofclaim 41, wherein a length of the proximal portion of the distal sectionof the lead body is selected from the group consisting of between about5 cm and about 15 cm, about 10 cm, between about 2 cm and about 17 cm,between about 7 cm and about 13 cm, and between about 9 cm and about 11cm.
 43. The medical electrical lead of claim 1, wherein the lead bodycomprises a material selected from the group consisting of siliconerubber, polyurethane, PEBAX, PTFE, and ETFE.
 44. The medical electricallead of claim 1, wherein the first and second segments comprise meansfor changing the bending stiffness of the lead body as a function ofaxial distance selected from the group consisting of coils havingvariable pitch as a function of axial distance, coils having variablewinding as a function of axial distance, coils having variable diameteras a function of axial distance, coils having variable pitch as afunction of axial distance, the lead body having variable diameter as afunction of axial distance, progressively adding more material to thelead body as a function of axial distance, adding more coils to the leadbody as a function of axial distance, varying lead body insulationthickness as a function of axial distance, varying lead body insulationtype as a function of axial distance, progressively incorporating morering-shaped members into the lead body as a function of axial distance,varying electrode structure as a function of axial distance, varyingelectrode positioning as a function of axial distance, including membershaving changing bending stiffness along an outside portion of the leadbody, disposing a member internally in the lead body having variablethickness as a function of axial distance, flattening portions of thelead body, and incorporating depressions into the lead body.
 45. Themedical electrical lead of claim 1, wherein the first and secondsegments comprise means for changing the bending stiffness of the leadbody as a function of axial distance x selected from the groupconsisting of varying the bending modulus as a function of axialdistance x of the material from which the lead body is formed, varyingthe density as a function of axial distance x of the material from whichthe lead body is formed, varying the composition as a function of axialdistance x of a polymer from which the lead body is formed, varying theamount of cross-linking as a function of axial distance x in a polymerfrom which the lead body is formed, varying the flexule moduli as afunction of axial distance x of the material from which the lead body isformed, varying the amount of a first polymer included, blended or mixedin a second polymer as a function of axial distance x, a shape-memoryalloy member capable of having its bending stiffness be varied throughselective activation of pre-determined portions thereof as a function ofaxial distance x, varying the composition of polymers included in thelead body as a function of axial distance x.
 46. The medical electricallead of claim 1, wherein the lead body is configured and dimensionedsuch that when the lead is implanted within the heart the first segmentis disposed in a distal portion of one of a great cardiac vein, a middlecardiac vein, a coronary sinus, a small cardiac vein, a posteriorcardiac vein, an oblique left atrial vein, and an anterior cardiac vein.47. The medical electrical lead of claim 1, wherein the lead body isconfigured and dimensioned such that when the lead is implanted withinthe heart the second segment is disposed in a distal portion of one of agreat cardiac vein, a middle cardiac vein, a coronary sinus, a smallcardiac vein, a posterior cardiac vein, an oblique left atrial vein, andan anterior cardiac vein.
 48. The medical electrical lead of claim 1,wherein the lead body and the at least one electrode are configured anddimensioned such that when the lead is appropriately implanted within agreat cardiac vein or a posterior cardiac vein of the heart a leftventricle of the heart may be electrically stimulated.
 49. The medicalelectrical lead of claim 1, wherein the lead body and the at least oneelectrode are configured and dimensioned such that when the lead isappropriately implanted within an oblique left atrail vein of the hearta left atrium of the heart may be electrically stimulated.
 50. Themedical electrical lead of claim 1, wherein the lead body and the atleast one electrode are configured and dimensioned such that when thelead IS appropriately implanted within a middle portion of a greatcardiac vein a right ventricle of the heart may be electricallystimulated.
 51. The medical electrical lead of claim 1, wherein the leadbody and the at least one electrode are configured and dimensioned suchthat when the lead is appropriately implanted within an anterior cardiacvein a left atrium of the heart may be electrically stimulated.
 52. Themedical electrical lead of claim 1, wherein the lead body and the atleast one electrode are configured and dimensioned such that when thelead is appropriately implanted within an anterior cardiac vein a leftventricle of the heart may be electrically stimulated.
 53. The medicalelectrical lead of claim 1, wherein the at least one electrode furthercomprises an anode and a cathode, and wherein the lead body and theanode and the cathode are configured and dimensioned such that when thelead is appropriately implanted within a middle cardiac vein electricalstimulation of apical portions of the heart may be effected.
 54. Themedical electrical lead of claim 1, wherein the at least one electrodefurther comprises an anode and a cathode, and wherein the lead body andthe anode and the cathode are configured and dimensioned such that whenthe lead is appropriately implanted within a posterior cardiac veinelectrical stimulation of basal portions of the heart may be effected.55. The medical electrical lead of claim 1, wherein the at least oneelectrode further comprises an anode and a cathode, and wherein the leadbody and the anode and the cathode are configured and dimensioned suchthat when the lead is appropriately implanted within a great cardiacvein electrical stimulation of basal portions of the heart may beeffected.
 56. The medical electrical lead of claim 1, wherein the distalsection of the lead body comprises a plurality of alternating firstsegments and second segments.
 57. The medical electrical lead of claim1, wherein at least one of the first segment and the second segment hasa length selected from the group consisting of about 8 mm, between about5 mm and about 10 mm, between about 5 mm and about 12 mm, and betweenabout 5 mm and about 50 mm.
 58. The medical electrical lead of claim 1,wherein the distal end is tapered.
 59. The medical electrical lead ofclaim 1, wherein at least a portion of the lead body has an outerdiameter selected from the group consisting of between about 1 mm andabout 2 mm, about 0.5 mm, about 3 mm, and exceeding 3 mm.
 60. Themedical electrical lead of claim 1, wherein the at least one electrodeis disposed in the first segment.
 61. The medical electrical lead ofclaim 1, wherein the distal section of the lead body comprises aplurality of alternating first segments and second segments and the atleast one electrode further comprises a cathode and an anode, the anodeand the cathode being disposed on different first segments.
 62. Themedical electrical lead of claim 1, wherein the at least one electrodefurther comprises a cathode and an anode, the anode and the cathodebeing disposed on the first segment.
 63. The medical electrical lead ofclaim 1, wherein the at least one electrode further comprises a cathodeand an anode, the anode and the cathode being separated from one anotheralong the lead body by a distance selected from the group consisting ofbetween about 4 mm and about 12 mm, between about 5 mm and about 10 mm,between about 5 mm and about 7 mm, and about 5 mm, between about 20 mmand about 50 mm, about 60 mm, and about 15 mm.
 64. The medical electrodeof claim 1, wherein the lead body further comprises a lumen formedtherein for accepting a stylet.
 65. The medical electrical lead of claim1, wherein the distal section of the lead body comprises a plurality offirst and second segmen ts, the first and second segments beingconfigured and dimensioned such that the lead body exhbits a number ofdifferent minimum mechanical energy storage positions the lead body mayassume within a venous anatomy of a patient.
 66. The medical electricallead of claim 65, wherein the first and second segments have first andsecond lengths, and wherein the first and second lengths are selectedaccording to the radii of different venous curves which are anticipatedto be encountered when the lead is implanted within the heart.
 67. Themedical electrical lead of claim 1, wherein the distal section of thelead body comprises a plurality of first and second segments havingfirst and second lengths, respectively, and wherein the second segmentsare configured and dimensioned to be located in or along at least afirst curve having a first radius of curvature in a venous anatomy ofthe heart, and wherein the first segments are configured and dimensionedto be located in or along a second curve having a second radiusofcurvature, the first radius being smaller than the second radius.
 68. Anelongated implantable medical electrical lead for electricallystimulating a human heart or sensing electrical signals originatingtherefrom, comprising: (a) an elongated lead body comprising proximaland distal sections, the elongated lead body defining axial distanceswhich increase distally; (b) at least one electrode for sensing orelectrically stimulating the heart; (c) a proximal end comprising anelectrical connector, the connector being contiguous with the proximalsection of the lead body; (d) a distal end portion contiguous with thedistal section of the lead body, the distal end portion extendingdistally from the distal section of the lead body; (e) at least oneelectrical conductor having proximal and distal ends, the distal end ofthe conductor being operatively connected to the at least one electrode,the proximal end of the conductor being operatively connected to theelectrical connector; wherein the distal section of the lead bodyfurther comprises a variable bending stiffness portion having bendingstiffnesses which increase in respect of axial distance.
 69. The medicalelectrical lead of claim 68, wherein the ratio of the bending stiffnessof a distal-most portion of the distal section (S_(bs)) in respect ofthe bending stiffness of a proximal-most portion of the distal section(S_(bf)) is defined by the equation:$1.5 \leq \frac{S_{bs}}{S_{bf}} \leq 100$


70. The medical electrical lead of claim 68, wherein the ratio of thebending stiffness of a distal-most portion of the distal section(S_(bs)) in respect of the bending stiffness of a proximal-most portionof the distal section (S_(bf)) is defined by the equation:$1.5 \leq \frac{S_{bs}}{S_{bf}} \leq 10$


71. The medical electrical lead of claim 65, wherein the bendingstiffness of a distal-most portion of the distal section (S_(bs)) isgreater than the bending stiffness of a proximal-most portion of thedistal section (S_(bf)) by a factor of at least about
 2. 72. The medicalelectrical lead of claim 65, wherein the bending stiffness of the distalsection of the lead body increases distally in one of step-wise,monotonic, exponential or logarithmic fashion.
 73. The medicalelectrical lead of claim 65, wherein the lead assumes a substantiallystraight shape prior to implantation.
 74. The medical electrical lead ofclaim 65, wherein the distal section of the lead body is formed into acurved configuration.
 75. The medical electrical lead of claim 65,wherein the distal section of the lead body is dimensioned andconfigured for implantation within a coronary sinus or cardiac vein ofthe heart.
 76. The medical electrical lead of claim 65, wherein afixation device is attached to the lead body.
 77. The medical electricallead of claim 76, wherein the fixation device is selected from the groupconsisting of a helical screw, a barb, a hook, at least one tine, and atleast one arm.
 78. The medical electrical lead of claim 65, wherein theat least one electrode is located along the distal section of the leadbody at a location appropriate to locate the electrode adjacent the leftatrium or left ventricle of the heart.
 79. The medical electrical leadof claim 65, wherein the lead, the at least one electrode, and distalsection of the lead body are configured and dimensioned to form a singlepass lead for dual chamber pacing of a left atrium and a left ventriclevia implantation within a coronary sinus and a great cardiac vein of theheart.
 80. The medical electrical lead of claim 65, wherein the leadbody is configured to permit preferential bending thereof along at leastone predetermined bending plane.
 81. The medical electrical lead ofclaim 65, wherein the bending stiffness of the distal section of thelead body is rotationally symmetric.
 82. The medical electrical lead ofclaim 65 wherein the bending stiffness of the distal section of the leadbody is rotationally asymmetric.
 83. The medical electrical lead ofclaim 82, wherein the at least one electrode and the lead body aredimensioned and configured such that when the lead is appropriatelyimplanted within a venous portion of the heart the rotationallyasymmetric segment may be employed by a physician to orient placement ofthe at least one electrode such that the electrode is pressed against ordirected towards a selected portion of the heart.
 84. The medicalelectrical lead of claim 65, wherein the lead has a unipolar electrodeconfiguration.
 85. The medical electrical lead of claim 65, wherein thelead has a multipolar electrode configuration.
 86. The medicalelectrical lead of claim 65, wherein the lead further comprises at leastone defibrillation electrode.
 87. The medical electrical lead of claim65, wherein the lead body further comprises a first asymmetriccross-section configured for implantation in a first preferredorientation in pre-determined distalmost portions of the heart's venousanatomy where bending radii are small, and a second asymmetriccross-section configured for implantation in a second preferredorientation different from the first orientation in pre-determinedportions of the heart's venous anatomy located proximal from thedistalmost portions thereof.
 88. The medical electrical lead of claim65, wherein the distal section of the lead body comprises means forchanging the bending stiffness of the lead body as a function of axialdistance selected from the group consisting of coils having variablepitch as a function of axial distance, coils having variable winding asa function of axial distance, coils having variable diameter as afunction of axial distance, coils having variable pitch as a function ofaxial distance, the lead body having variable diameter as a function ofaxial distance, progressively adding more material to the lead body as afunction of axial distance, adding more coils to the lead body as afunction of axial distance, varying lead body insulation thickness as afunction of axial distance, varying lead body insulation type as afunction of axial distance, progressively incorporating more ring-shapedmembers into the lead body as a function of axial distance, varyingelectrode structure as a function of axial distance, varying electrodepositioning as a function of axial distance, including members havingchanging bending stiffness along an outside portion of the lead body,disposing a member internally in the lead body having variable thicknessas a function of axial distance, flattening portions of the lead body,and incorporating depressions into the lead body.
 89. The medicalelectrical lead of claim 65, wherein the distal section of the lead bodycomprises means for changing the bending stiffness of the lead body as afunction of axial distance x selected from the group consisting ofvarying the bending modulus as a function of axial distance x of thematerial from which the lead body is formed, varying the density as afunction of axial distance x of the material from which the lead body isformed, varying the composition as a function of axial distance x of apolymer from which the lead body is formed, varying the amount ofcross-linking as a function of axial distance x in a polymer from whichthe lead body is formed, varying the flexule moduli as a function ofaxial distance x of the material from which the lead body is formed,varying the amount of a first polymer included, blended or mixed in asecond polymer as a function of axial distance x, a shape-memory alloymember capable of having its bending stiffness be varied throughselective activation of pre-determined portions thereof as a function ofaxial distance x, varying the composition of polymers included in thelead body as a function of axial distance x.
 90. The medical electricallead of claim 65, wherein the distal section of the lead body has alength selected from the group consisting of about 8 mm, between about 5mm and about 10 mm, between about 5 mm and about 12 mm, and betweenabout 5 mm and about 50 mm.
 91. The medical electrical lead of claim 65,wherein the distal end portion is tapered.
 92. The medical electricallead of claim 65, wherein at least a portion of the lead body has anouter diameter selected from the group consisting of between about 1 mmand about 2 mm, about 0.5 mm, about 3 mm, and exceeding 3 mm.
 93. Themedical electrical lead of claim 65, wherein the at least one electrodefurther comprises a cathode and an anode.
 94. The medical electricallead of claim 65, wherein the at least one electrode further comprises acathode and an anode, the anode and the cathode being separated from oneanother along the lead body by a distance selected from the groupconsisting of between about 4 mm and about 12 mm, between about 5 mm andabout 10 mm, between about 5 mm and about 7 mm, and about 5 mm, betweenabout 20 mm and about 50 mm, about 60 mm, and about 15 mm.
 95. Themedical electrode of claim 65, wherein the lead body further comprises alumen formed therein for accepting a stylet.
 96. A system forelectrically stimulating, or sensing electrical signals originatingfrom, a human heart, the system comprising: (a) an implantable cardiacstimulator, and (b) an elongated implantable medical electrical lead forelectrically stimulating the heart or sensing electrical signalsoriginating therefrom, comprising: (i) a lead body having proximal anddistal sections; (ii) at least one electrode for sensing or electricallystimulating the heart; (iii) a proximal end comprising an electricalconnector, the electrical connector being contiguous with the proximalsection of the lead body, the electrical connector being configured foroperative attachment to the cardiac stimulator; (iv) a distal endcontiguous with the distal section of the lead body; (v) at least oneelectrical conductor having proximal and distal ends, the distal end ofthe conductor being operatively connected to the at least one electrode,the proximal end of the conductor being operatively connected to theelectrical connector; wherein the distal section of the lead bodycomprises at least first and second segments, the first segment having abending stiffness S_(bs) which exceeds the bending stiffness S_(bf) ofthe second segment, the first and second segments being configured anddimensioned to impart a distally directed force to the distal end of thelead when the first and second segments are subjected to a bendingmoment resulting in a sufficient curvature of the distal section of thelead body.
 97. The system of claim 96, wherein the cardiac stimulator isselected from the group consisting of a pacemaker, an implantable pulsegenerator (IPG), an implantable cardioverter-defibrillator (ICD), apacer-cardioverter-defibrillator (PCD), and an implantabledefibrillator.
 98. A system for electrically stimulating, or sensingelectrical signals originating from, a human heart, the systemcomprising: (a) an implantable cardiac stimulator, and (b) an elongatedimplantable medical electrical lead for electrically stimulating theheart or sensing electrical signals originating therefrom, comprising:(i) an elongated lead body comprising proximal and distal sections, theelongated lead body defining axial distances which increase distally;(ii) at least one electrode for sensing or electrically stimulating theheart; (iii) a proximal end comprising an electrical connector, theconnector being contiguous with the proximal section of the lead body;(iv) a distal end portion contiguous with the distal section of the leadbody, the distal end portion extending distally from the distal sectionof the lead body; (v) at least one electrical conductor having proximaland distal ends, the distal end of the conductor being operativelyconnected to the at least one electrode, the proximal end of theconductor being operatively connected to the electrical connector;wherein the distal section of the lead body further comprises a variablebending stiffness portion having bending stiffnesses which increase inrespect of axial distance.
 99. The system of claim 98, wherein thecardiac stimulator is selected from the group consisting of a pacemaker,an implantable pulse generator (IPG), an implantablecardioverter-defibrillator (ICD), a pacer-cardioverter-defibrillator(PCD), and an implantable defibrillator.
 100. A method of electricallystimulating a patient's heart with an implantable cardiac stimulator andan elongated implantable medical electrical lead, the lead comprising alead body having proximal and distal sections, at least one electrodefor sensing or electrically stimulating the heart, a proximal endcomprising an electrical connector, the electrical connector beingcontiguous with the proximal section of the lead body, the electricalconnector being configured for operative attachment to the cardiacstimulator, a distal end contiguous with the distal section of the leadbody, at least one electrical conductor having proximal and distal ends,the distal end of the conductor being operatively connected to the atleast one electrode, the proximal end of the conductor being operativelyconnected to the electrical connector, the distal section of the leadbody comprising at least first and second segments, the first segmenthaving a bending stiffness S_(bs) which exceeds the bending stiffnessS_(bf) of the second segment, the first and second segments beingconfigured and dimensioned to impart a distally directed force to thedistal end of the lead when the first and second segments are subjectedto a bending moment resulting in a sufficient curvature of the distalsection of the lead body, the method comprising: (a) providing thecardiac stimulator; (b) providing the medical electrical lead; (c)transvenously inserting and positioning the lead through a coronarysinus and into a coronary vein in the the heart, (d) operativelyconnecting the connector of the lead to the cardiac stimulator; and (e)delivering at least one electrical pulse originating in the cardiacstimulator through the lead and the at least one electrode to the heart.101. The method of claim 100, wherein the at least one electrical pulseis a pacing pulse, the method further comprising delivering a pacingpulse to the heart.
 102. The method of claim 100 wherein the at leastone electrical pulse is a defibrillation pulse, the method furthercomprising delivering a pacing pulse to the heart.
 103. The method ofclaim 100, the method further comprising employing astylet wheninserting and positioning the lead in the heart.
 104. The method ofclaim 100, the method further comprising employing a guide catheter whenintroducing the lead into the coronary sin us.
 105. The method of claim100, the method further comprising removing the guide catheter after thelead has been inserted through the coronary sinus.
 106. A method ofelectrically stimulating a patient's heart with an implantable cardiacstimulator and an elongated implantable medical electrical lead, thelead comprising an elongated lead body comprising proximal and distalsections, the elongated lead body defining axial distances whichincrease distally, at least one electrode for sensing or electricallystimulating the heart, a proximal end comprising an electricalconnector, the connector being contiguous with the proximal section ofthe lead body, a distal end portion contiguous with the distal sectionof the lead body, the distal end portion extending distally from thedistal section of the lead body, at least one electrical conductorhaving proximal and distal ends, the distal end of the conductor beingoperatively connected to the at least one electrode, the proximal end ofthe conductor being operatively connected to the electrical connector,the distal section of the lead body further comprising a variablebending stiffness portion having bending stiffnesses which increase inrespect of axial distance, the method comprising: (a) providing thecardiac stimulator; (b) providing the medical electrical lead; (c)transvenously inserting and positioning the lead through a coronarysinus and into a coronary vein in the the heart, (d) operativelyconnecting the connector of the lead to the cardiac stimulator; and (e)delivering at least one electrical pulse originating in the cardiacstimulator through the lead and the at least one electrode to the heart.107. The method of claim 106, wherein the at least one electrical pulseis a pacing pulse, the method further comprising delivering a pacingpulse to the heart.
 108. The method of claim 106, wherein the at leastone electrical pulse is a defibrillation pulse, the method furthercomprising delivering a pacing pulse to the heart.
 109. The method ofclaim 106, the method further comprising employing a stylet wheninserting and positioning the lead in the heart.
 110. The method ofclaim 106, the method further comprising employing a guide catheter whenintroducing the lead into the coronary sinus.
 111. The method of claim110, the method further comprising removing the guide catheter after thelead has been inserted through the coronary sinus.
 112. A method ofelectrically stimulating a patient's heart with an implantable cardiacstimulator and an elongated implantable medical electrical lead, thelead comprising a lead body having proximal and distal sections, atleast one electrode for sensing or electrically stimulating the heart, aproximal end comprising an electrical connector, the connector beingcontiguous with the proximal section of the lead body, a distal endconnected to the distal section of the lead body, at least oneelectrical conductor having proximal and distal ends, the distal end ofthe conductor being operatively connected to the at least one electrode,the proximal end of the conductor being operatively connected to theelectrical connector, the distal section of the lead body comprising atleast first and second segments, the first segment having a bendingstiffness S_(b1), the second segment having a bending stiffness S_(b2),S_(b1) not equalling S_(b2), the first segment, and the second segmentbeing configured and characterized such that a distally directed forceis imparted to the distal end of the lead when the first and secondsegments are subjected to a bending moment resulting in a sufficientcurvature of the distal section of the lead body, the bending momentbeing provided by an external force applied to the lead, the methodcomprising: (a) providing the cardiac stimulator; (b) providing themedical electrical lead; (c) transvenously inserting and positioning thelead through a coronary sinus and into a coronary vein in the the heart,(d) operatively connecting the connector of the lead to the cardiacstimulator; and (e) delivering at least one electrical pulse originatingin the cardiac stimulator through the lead and the at least oneelectrode to the heart.
 113. The method of claim 112, wherein the atleast one electrical pulse is a pacing pulse, the method furthercomprising delivering a pacing pulse to the heart.
 114. The method ofclaim 112, wherein the at least one electrical pulse is a defibrillationpulse, the method further comprising delivering a pacing pulse to theheart.
 115. The method of claim 112, the method further comprisingemploying a stylet when inserting and positioning the lead in the heart.116. The method of claim 112, the method further comprising employing aguide catheter when introducing the lead into the coronary sinus. 117.The method of claim 116, the method further comprising removing theguide catheter after the lead has been inserted through the coronarysinus.
 118. A method of making an elongated implantable medicalelectrical lead, the lead comprising a lead body having proximal anddistal sections, at least one electrode for sensing or electricallystimulating the heart, a proximal end comprising an electricalconnector, the electrical connector being contiguous with the proximalsection of the lead body, the electrical connector being configured foroperative attachment to the cardiac stimulator, a distal end contiguouswith the distal section of the lead body, at least one electricalconductor having proximal and distal ends, the distal end of theconductor being operatively connected to the at least one electrode, theproximal end of the conductor being operatively connected to theelectrical connector, the distal section of the lead body comprising atleast first and second segments, the first segment having a bendingstiffness S_(bs) which exceeds the bending stiffness S_(bf) of thesecond segment, the first and second segments being configured anddimensioned to impart a distally directed force to the distal end of thelead when the first and second segments are subjected to a bendingmoment resulting in a sufficient curvature of the distal section of thelead body, the method comprising: (a) providing the at least oneelectrode; (b) providing the at least one electrical conductor; (c)providing the electrical connector; (d) operatively connecting theelectrical connector to the proximal end of the electrical conductor;(e) operatively connecting the distal end of the electrical conductor tothe at least one electrode; (f) providing the lead body; and (g)incorporating the at least one electrical conductor, the at least oneelectrode, the electrical connector and the lead body into the lead.119. A method of making an elongated implantable medical electricallead, the lead comprising an elongated lead body comprising proximal anddistal sections, the elongated lead body defining axial distances whichincrease distally, at least one electrode for sensing or electricallystimulating the heart, a proximal end comprising an electricalconnector, the connector being contiguous with the proximal section ofthe lead body, a distal end portion contiguous with the distal sectionof the lead body, the distal end portion extending distally from thedistal section of the lead body, at least one electrical conductorhaving proximal and distal ends, the distal end of the conductor beingoperatively connected to the at least one electrode, the proximal end ofthe conductor being operatively connected to the electrical connector,the distal section of the lead body further comprising a variablebending stiffness portion having bending stiffnesses which increase inrespect of axial distance, the method comprising: (a) providing the atleast one electrode; (b) providing the at least one electricalconductor; (c) providing the electrical connector; (d) operativelyconnecting the electrical connector to the proximal end of theelectrical conductor; (e) operatively connecting the distal end of theelectrical conductor to the at least one electrode; (f) providing thelead body; and (g) incorporating the at least one electrical conductor,the at least one electrode, the electrical connector and the lead bodyinto the lead.
 120. A method of making an elongated implantable medicalelectrical lead, the lead comprising a lead body having proximal anddistal sections, at least one electrode for sensing or electricallystimulating the heart, a proximal end comprising an electricalconnector, the connector being contiguous with the proximal section ofthe lead body, a distal end connected to the distal section of the leadbody, at least one electrical conductor having proximal and distal ends,the distal end of the conductor being operatively connected to the atleast one electrode, the proximal end of the conductor being operativelyconnected to the electrical connector, wherein the distal section of thelead body comprises at least first and second adjoining segments, thefirst segment being relatively stiff, the second segment beingrelatively flexible, the first and second segments being configured toimpart a distally directed force to the distal end of the lead when thesegments are subjected to a bending moment resulting in a sufficientcurvature of the distal section of the lead body, the method comprising:(a) providing the at least one electrode; (b) providing the at least oneelectrical conductor; (c) providing the electrical connector; (d)operatively connecting the electrical connector to the proximal end ofthe electrical conductor; (e) operatively connecting the distal end ofthe electrical conductor to the at least one electrode; (f) providingthe lead body; and (g) incorporating the at least one electricalconductor, the at least one electrode, the electrical connector and thelead body into the lead.
 121. A method of making an elongatedimplantable medical electrical lead, the lead comprising a lead bodyhaving proximal and distal sections, at least one electrode for sensingor electrically stimulating the heart, a proximal end comprising anelectrical connector, the connector being contiguous with the proximalsection of the lead body, a distal end connected to the distal sectionof the lead body, at least one electrical conductor having proximal anddistal ends, the distal end of the conductor being operatively connectedto the at least one electrode, the proximal end of the conductor beingoperatively connected to the electrical connector, the distal section ofthe lead body comprising at least first and second segments, the firstsegment having a bending stiffness S_(b1), the second segment having abending stiffness S_(b2), S_(b1) not equalling S_(b2), the first segmentand the second segment being configured and characterized such that adistally directed force is imparted to the distal end of the lead whenthe first and second segments are subjected to a bending momentresulting in a sufficient curvature of the lead body, the bending momentbeing provided by an external force applied to the lead, the methodcomprising: (a) providing the at least one electrode; (b) providing theat least one electrical conductor; (c) providing the electricalconnector; (d) operatively connecting the electrical connector to theproximal end of the electrical conductor; (e) operatively connecting thedistal end of the electrical conductor to the at least one electrode;(f) providing the lead body; and (g) incorporating the at least oneelectrical conductor, the at least one electrode, the electricalconnector and the lead body into the lead.